Anytime is a good time to buy the R3. Prices will continue to drop over time. I paid 2825 brand new from GT back in January and I think can be had cheaper now too. I'm not sure what it would take but Sony would have to do something dramatic to get me to upgrade. I'm pretty confident I'll keep the R3 for a good 3-4 more years and go ahead and skip the R4 and wait for the R5.
I think the biggest improvements they will work on is FPS/Video features. Maybe a swivel out sideways LCD like canon. This would be pretty huge actually.